Depositional ice nucleation on solid ammonium sulfate and glutaric acid particlesK. J. Baustian1,2, M. E. Wise1,3, and M. A. Tolbert1,31Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA2Department of Atmospheric and Oceanic Science, University of Colorado, Boulder, Colorado, USA3Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, USA

Abstract. Heterogeneous ice nucleation on solid ammonium sulfate and glutaric acid
particles was studied using optical microscopy and Raman spectroscopy.
Optical microscopy was used to detect selective nucleation events as water
vapor was slowly introduced into an environmental sample cell. Particles
that nucleated ice were dried via sublimation and examined in detail using
Raman spectroscopy. Depositional ice nucleation is highly selective and
occurred preferentially on just a few ammonium sulfate and glutaric acid
particles in each sample. For freezing temperatures between 214 K and 235 K
an average ice saturation ratio of S = 1.10±0.07 for solid ammonium
sulfate was observed. Over the same temperature range, S values observed for
ice nucleation on glutaric acid particles increased from 1.2 at 235 K to 1.6
at 218 K. Experiments with externally mixed particles further show that
ammonium sulfate is a more potent ice nucleus than glutaric acid.
Our results suggest that heterogeneous nucleation on ammonium sulfate may be
an important pathway for atmospheric ice nucleation and cirrus cloud
formation when solid ammonium sulfate aerosol
particles are available for ice formation. This pathway for ice formation
may be particularly significant near the tropical tropopause region where
sulfates are abundant and other species known to be good ice nuclei are
depleted.